Electromagnetic interference (EMI) is a well known problem with electronic instruments having circuitry operating at high frequencies. Uncontrolled EMI from one instrument may interfere with the operation of other nearby sensitive electronic instruments. Accordingly, EMI has been governmentally regulated to limit the amount of tolerated EMI emissions.
To control, limit, or avoid EMI emissions, instruments are often shielded by use of metal housings. Housing openings are limited in size to limit the wavelengths of signals that may be emitted. Seams or apertures at which doors, panels, and lids are installed are provided with conductive edge springs or gaskets to electrically "close" or shorten the length of any elongated gaps.
EMI has been well known in high speed instrumentation operating in the range of several Gigahertz, where shielding must close any gaps more than about 1 cm. With the generally high value of such instruments, the cost to provide adequate shielding has been tolerable. However, low cost personal computers and other consumer appliances use increasingly high speed processors that make cost effective EMI control an important concern.
One particular area of concern in personal computers is the array of I/O apertures that provide access to the removable I/O cards. These include modems, video circuits, and sound cards. Each card has a row of conductive pads on one major edge that is received by an edge connector on the mother board of the computer, and has a perpendicular metal plate at one end of the card that covers an I/O aperture when the card is installed. A connector mounted to the plate is normally connected to a cable that extends to a device outside of the computer.
As I/O apertures are about 10 cm long, the gap between the I/O card plate and the apertures in the housing panel requires shielding. Conventional metal spring gaskets have proven unsuitable because the tight space constraints between apertures prevent the use of conventional fasteners. With the standard 0.8" (20.3 mm) pitch between apertures, and the standard 0.73" (18.5 mm) plate width, there is only a gap of 0.07" (1.8 mm) between the edges of properly positioned adjacent I/O card plates. This is inadequate for the heads of conventional fasteners such as screws and rivets. Because the plates must be pressed flush against the housing panel, protrusions must be limited to the narrow gap between plates.
To avoid oversize protrusions, adhesive attachment of spring gaskets and conductive elastomeric gaskets have been proposed, but these lack the durability required for repeated insertions and removals of I/O cards, particularly by inexperienced users. Adhesives have a limited life span, and soft materials are susceptible to tearing and other damage. In addition, these proposed approaches lack the means to prevent the misalignment of I/O cards that may wobble in their edge connector.
The embodiments disclosed herein overcome these limitations by providing an instrument housing that defines a chamber and has a metal shell with an aperture region defining an array of parallel elongated apertures. The shell has an elongated web between each pair of adjacent apertures, and a raised element protrudes from the web. The raised element is spaced apart from each aperture to reveal a portion of the web near each aperture. A resilient, electrically conductive spring element is electrically connected to the web and has a compressible portion extending away from the web, such that a metal plate pressed toward the revealed surface makes electrical contact with the spring and thereby to the shell, and is laterally constrained by the raised element. The raised element may be welded to the web with the spring captured between, or the raised element may be partially sheared to protrude from the web, with an aperture in the spring closely receiving the raised portion with an interference fit.